When two communication devices simultaneously transmit on the same channel, a communication conflict might occur, potentially degrading the receiving probability of one or both of these transmissions. Such communication conflicts are obviously undesired, yet cannot be disregarded in the crowded communication networks which are usually short in bandwidth.
The penalty for such transmission collisions is a lower quality of service, more power consumption and more undesired RF radiation. Therefore, multiple access communication networks employ methods to properly allocate communication channels to devices, in order to avoid such conflicts. Still, due to the high ratio of devices per channel, and to the often unsynchronized transmissions, among different devices and users that share these channels, such conflicts are still an important issue to consider in communication systems.
One communication sector particularly vulnerable to channel allocation conflicts is related to networks comprising a multitude of one-way transmitters that share a relatively small amount of channels. In such networks, transmitters cannot coordinate with each other the allocation of channels, so there is a chance that two such devices will simultaneously transmit on the same channel and interfere with each other. One such particular network that employs millions of one-way communication devices sharing thirty three or less channels is Cospas-Sarsat. Though the scope of the present invention is far beyond that specific system, Cospas-Sarsat is a good example to clarify the need and solution and a particular embodiment disclosed by the present invention, so is specifically enlightened here.
Cospas-Sarsat is a satellite communications system to assist Search and Rescue (SAR) of people in distress, all over the world and at anytime. The system was launched in 1982 by the USA, Canada, France and the Soviet Union (now Russia) and since then, it has been used for thousands of SAR events and has been instrumental in the rescue of over 20,000 lives worldwide. The goal of the system is to detect and locate signals from distress radio beacons and forward these data to ground stations, in order to support all organizations in the world with responsibility for SAR operations, whether at sea, in the air or on land. The system uses spacecraft—Low Earth Orbit (LEO); Geostationary (GEO) satellites; and in the future also Medium Earth Orbit (MEO); as well as ground facilities. Cospas-Sarsat radio beacons transmit in the 406 MHz band (and 121.5 MHz until 2009). The position of the beacon is determined either by the Doppler shift of the received beacon signal or by position data provided by an embedded Global Navigation Satellite System (GNSS) decoder (receiver), integrated with the radio beacon. A detailed description of the Cospas-Sarsat System is provided in the document “Introduction to the Cospas-Sarsat System, C/S G.003”, which can be accessed through the following link—http://cospas-sarsat.org/Documents/gDocs.htm.
Several types of Cospas-Sarsat beacons are approved for use, differing mainly in their mechanical structure and activation method, customized for different applications: a) Emergency Position Indicating Radio Beacon (EPIRB) for marine use; b) Emergency Locator Transmitter (ELT) for aviation use; and c) Personal Locator Beacon (PLB) for personal and/or terrestrial use.
Cospas-Sarsat beacons are deployed in large quantities, all over the world, sharing a few transmission channels. Further, these distress beacons are activated upon local triggering and their transmissions are not synchronized in time, neither in frequency, with each other (except of a very rough control through a preliminary factory set frequency allocation). A narrow bandwidth is allocated to all these beacons which are usually off, typically for several years, and transmit only in rare occasions, for a short time, normally in periodic bursts for several days. However, as tens and hundreds of thousands of such beacons are deployed, sharing one narrowband channel, simultaneous transmissions might statistically occur, interfering with each other and decreasing the probability of a distress message to be detected. Then, in order to ensure a certain quality of service, i.e. a minimal probability for a distress message to be detected within a specific period of time, the number of transmitters per channel should be limited. Obviously, such a system could be more efficient in exploitation of the allocated spectrum, serve more beacons and/or improve the quality of service if the transmission collision rate could be reduced.
The total bandwidth allocated for Cospas-Sarsat is 100 KHz (406.0-406.1 MHz), divided to 3 KHz bandwidth channels. According to the present art, each beacon is factory set to one of these channels, which cannot be reconfigured in the field. Then, each channel (theoretically 33 channels, however practically much less mainly due to Doppler shift limitations and system overhead) is shared by tens or hundreds of thousands of beacons. When activated (automatically or manually), a Cospas-Sarsat beacon transmits short bursts, each one approximately 0.5 seconds long, every 50 seconds, for several days, until its battery drains. In order to avoid repetitive collisions between two active beacons, a beacon is required to set its transmission cycle to 50+/−2.5 seconds, and the period should be randomized around a mean value of 50 seconds, so that time intervals between transmissions are randomly distributed on the interval 47.5 to 52.5 seconds.
A significant augmentation of the Cospas-Sarsat satellite segment is planed to be implemented in the near future. Accordingly, compatible Cospas-Sarsat payloads will be installed onboard positioning satellites belonging to the Galileo GNSS constellation and possibly also onboard GPS satellites. Galileo, the upcoming European GNSS, is planed to comprise 27 satellites, while the US GPS comprises 24 operational satellites. Each of these systems provides at least four satellites simultaneously in Line of Sight (LOS) with any point on earth, as required for trilateral positioning (fourth satellite usually solves clock ambiguity), i.e. any point on earth will be always in at least four different satellite footprints (service areas) of the Galileo GNSS and another four GPS satellite footprints. Considering also the LEO and GEO satellites that currently cover the earth for this SAR system, it is expected that any beacon on earth will be at any moment in ten or more footprints, which move quickly relatively to the earth surface. Hence, many different intersections of footprints will be introduced on earth surface, enabling beacons on different intersections to communicate with different satellites. For the clarity of this discussion, it is assumed that beacons are featured with omnidirectional antennas, and satellites are installed with wide beam antennas, yet this is definitely not mandatory.
The applicant has proposed a method to improve channel allocation for communication networks, such as Cospas-Sarsat, in “Increasing Channel Capacity of TDMA Transmitters in Satellite based Networks”, application Ser. No. 12/046,509, filed on 12 Mar. 2008. This reference shares with the present invention the aspect of allocating time slots to Time Division Multiple Access (TDMA) transmitters based on their position, yet does not discuss allocation of frequencies and does not address overlapping of satellite footprints, specifically dynamically moving, and a cellular partitioning based on that.
Another aspect of communications vulnerable to channel allocation conflicts is the initial approach of a device to an access point asking for service. Often, a communications network properly allocates operational channels to devices, and well synchronizes these devices in order to avoid collisions. However, before the network allocates these operational channels, devices that initiate a service request do not use said operational channels, which are controlled by the network (or by any administering unit related to that network), but share a pool of service requesting channels. At this preliminary phase, these devices might not be synchronized with each other, for different reasons, such as: random timing of access, no peer to peer connection; communication peaks; etc. Improving the channel allocation method for devices requesting service, could enable a faster reaction of the network to such requests, and/or managing more devices requesting service, over the same channels/bandwidth.
U.S. Pat. No. 6,115,371 to Berstis (IBM) discloses a satellite uplink separation using time multiplexed global positioning system cell location beacon system. This method, for allocating bandwidth to devices seeking to initiate contact with a communication service, suggests using time slots according to self location determined by GPS.
U.S. Pat. No. 7,304,963 to Amouris discloses a method and system for dynamically allocating a set of broadcast TDMA channels to a network of transceiver nodes. This method is based on timeslot partitioning and geographic location.
U.S. Pat. No. 7,082,111 to Amouris discloses a method and system for dynamically allocating time slots of a common TDMA broadcast channel to a network of transceiver nodes. This invention allocates time slots to TDMA devices according to their geographical position.
Yet, none of these three US patents addresses intersections of footprints, i.e. areas served by several satellites, and neither Berstis nor Amouris suggests discriminating between devices placed in areas served by a different number of satellites, for channel allocation purposes.
U.S. Pat. No. 5,268,694 to Jan et al. (MOTOROLA) discloses a method of reusing spectrum on an approximately spherical surface, based on two satellite footprints partially overlapping, each footprint divided to cells. According to Jan, cells located in the intersection of footprints are defined non active, and channels are assigned only to active cells, spacing co-channel cells a predetermined distance apart. Yet, Jan does not address cells contained in other than one or two footprints, and neither suggests allocating active transmission channels to devices in overlapping footprints, specifically not according to the number of overlapping footprints.
WO/2001/095522 to Yung, Hagen and Chang, (HUGHES ELECTRONICS CORPORATION) discloses a RESOURCE ALLOCATION METHOD IN A SATELLITE DIVERSITY SYSTEM. This invention teaches allocating system resources to user terminals communicating with a multiple of satellites, wherein a ground hub compensates for differential propagation delays to any one of these remote users. Yet, this invention does not consider the various satellite footprint intersections as a basis for resource allocation.
European Patent EP0935351 to Bains, Navjit Singh (ICO Services) discloses a Radio resource management in a mobile satellite telephone system. This method teaches allocating radio resources to a plurality of mobile user terminals in a satellite mobile telephone system, in which a position of each of the user terminals within the footprint of a given satellite is capable of being classified, specifically denying resources from terminals placed at the edge of the satellite footprint, which obtain a significant path delay for a signal to be communicated to the given satellite. Still, this method does not address intersections of footprints for the purpose of resource allocation.
The present art methods described above have not yet provided satisfactory solutions to the problem of allocating communication channels to devices configured to transmit bursts of data to a diversity of satellites, specifically non geostationary satellites, sharing relatively few channels, particularly when having a certain amount of data transmission redundancy.
It is an object of the present invention to provide a system and method for allocating communication channels to devices configured to communicate with a diversity of satellites, particularly non geostationary satellites, reducing transmission collisions and exploiting the allocated bandwidth.
It is also an object of the present invention to provide a system and method for allocating communication channels to devices configured to communicate with non geostationary satellites, particularly devices that have no means to communicate with each other or cannot coordinate channel allocation among them.
It is another object of the present invention to provide a system and method for allocating momentary communication channels to devices configured to communicate with non geostationary satellites, in particular devices which are distress radio beacons.
It is yet another object of the present invention to provide a method for improving present or/and future systems for Search and Rescue (SAR), such as Cospas-Sarsat and Galileo.
It is as well an object of the present invention to provide a system and method for allocating momentary communication channels to devices configured to communicate with non geostationary satellites, based on the various intersections of overlapping footprints of satellites on the earth surface.
It is still an object of the present invention to provide a system and method for allocating communication channels to devices configured to communicate with non geostationary satellites, based on time synchronization and positioning information provided by a GNSS such as GPS or Galileo or GLONASS.
It is also an object of the present invention to provide a system and method for allocating communication channels to devices configured to communicate with non geostationary satellites, based on additional data such as statistics of geographical distribution of devices, redundancy of data, location variation and acknowledgement of transmissions.
It is still another object of the present invention to provide a system and method for allocating communication channels to devices configured to communicate with non geostationary satellites, minimizing cost and size and power consumption of said devices.
It is nonetheless an object of the present invention to provide an apparatus and method for allocating communication channels to devices configured to communicate with non geostationary satellites, wherein said channels are either time slots for Time Division Multiple Access (TDMA); or frequencies for Frequency Division Multiple Access (FDMA); or digital codes for Code Division Multiple Access (CDMA); or a combination thereof.
Other objects and advantages of the invention will become apparent as the description proceeds.